This invention relates to pressure sensitive adhesives and tape articles prepared therefrom. The tapes are characterized by exhibiting an overall balance of adhesive and cohesive characteristics and exceptional load bearing capabilities at elevated temperatures.
Pressure sensitive tapes are virtually ubiquitous in the home and workplace. In its simplest configuration, a pressure sensitive tape comprises an adhesive and a backing, and the overall construction is tacky at the use temperature and adheres to a variety of substrates using only moderate pressure to form the bond. In this fashion, pressure sensitive tapes constitute a complete, self-contained bonding system.
According to D. W. Aubrey in xe2x80x9cDevelopments in Adhesivesxe2x80x9d (ed. W. C. Wake, Vol. 1, Chapter 5, Applied Science Publishers: London, 1977), a good pressure sensitive adhesive (psa) must fulfill three main technical requirements:
1. The adhesive must conform quickly to the surface to which it is applied in order to rapidly form a bond; this relates to tack.
2. The adhesive must display adequate resistance to separation by peeling once the bond is formed; this relates to adhesion.
3. The adhesive must exhibit resistance to shear under the influence of an applied load; this relates to the adhesive""s cohesion.
These three requirements are assessed generally by means of tests which are designed to individually measure tack, adhesion (peel strength), and cohesion (shear holding power). These measurements taken together constitute the balance of properties often used to characterize a psa.
With broadened use of pressure sensitive tapes over the years, performance requirements have become more demanding. Shear holding capability, for example, which originally was intended for applications supporting modest loads at room temperature has now increased substantially for many applications in terms of operating temperature and load. So-called high performance pressure sensitive tapes are those capable of essentially permanently supporting loads of  greater than 300 grams per square centimeter of adhesive at temperatures of 150xc2x0 F. (65xc2x0 C.) or higher. Increased shear holding capability has generally been accomplished by crosslinking the psa, although considerable care must be exercised so that high levels of tack and adhesion are retained in order to retain the aforementioned balance of properties.
In one aspect this invention provides a novel pre-adhesive syrup polymer composition comprising:
a first component comprising a solute polymer comprising a plurality of polymerized monomer units comprising pendant reactive nucleophilic or electrophilic functional groups;
a second component having a plurality of co-reactive nucleophilic or electrophilic functional groups selected from a second solute polymer comprising a plurality of polymerized monomer units comprising co-reactive functional groups and a polyfunctional compound having co-reactive functional groups; and
a third component comprising at least one free-radically polymerizable solvent monomer.
The novel pre-adhesive syrup polymer compositions of the present invention cure to pressure sensitive adhesives possessing high load bearing capability at elevated temperatures by means of the photopolymerization of the solvent monomer component and crosslinking by means of reactive and co-reactive functional groups. In another aspect the invention provides an adhesive article comprising the polymerized, crosslinked syrup polymer coated on a substrate.
In another aspect this invention provides a process of preparing a novel adhesive composition which comprises the steps of providing the novel syrup polymer composition of this invention further comprising an effective amount of a photoinitiator, and subjecting said composition to sufficient energy to activate said photoinitiator to polymerize the solvent monomer(s) of the syrup polymer composition, and crosslinking the first solute polymer and second component by forming covalent bonds between the reactive and co-reactive functional groups.
For performance, environmental, and economic considerations, photoinitiated polymerization is a particularly desirable method for preparing a psa directly on the tape backing (or release liner in the case of a so-called transfer tape in which the psa is ultimately transferred to a substrate instead of a tape backing to provide for adhesion of the bonded article or adherend). With this bulk polymerization technique, a common practice in order to achieve a coatable viscosity of 500-10,000 centipoises is to partially polymerize, either thermally or photochemically, the monomers to a conversion of 5-10%.
In another aspect this invention provides a process for preparing an adhesive article comprising coating the novel syrup polymer composition on a substrate in the presence of a free-radical initiator, and subjecting the coated substrate to sufficient energy to polymerize the solvent monomer and crosslink the components by forming covalent bonds between the reactive and co-reactive functional groups.
Briefly, the present invention provides novel pressure sensitive photoadhesive compositions prepared from a first solute polymer containing reactive functional groups capable of reaction at effective rates (at normal processing temperatures) with a co-reactive second component possessing functionality that is complementary to that of the first solute polymer. By complementary is meant that if the solute polymer reactive functional groups are electrophilic in nature, the second component should possess co-reactive nucleophilic groups. The converse is also useful; when the solute polymer contains reactive nucleophilic groups then the second component contains co-reactive electrophilic groups. In addition, reactions involving polymeric reactants of the instant invention are controlled and precise in that they result in polymerxe2x80x94polymer coupling reactions only by reaction between the reactive and co-reactive functional groups. The polymerization of the novel syrup polymer composition has been discovered to provide high load holding capability pressure sensitive adhesives, especially at elevated temperatures.
In this application xe2x80x9cpre-adhesivexe2x80x9d refers to the solution comprising functional solute polymer, a second component and third monomer component which may be polymerized and crosslinked to form a pressure sensitive adhesive. xe2x80x9cSyrup polymerxe2x80x9d refers to a solution of a solute polymer in one or more solvent monomers, the solution having a viscosity of from 500 to 10,000 cPs at 22xc2x0 C.
The present invention provides pressure sensitive adhesives possessing essentially permanent, high load bearing capabilities at temperatures up to 70xc2x0 C. The pressure sensitive adhesives of the invention are polymers exhibiting a glass transition temperature of less than xe2x88x9215xc2x0 C. and are formed from 100 parts of ethylenically unsaturated monomers and polymers. The pressure sensitive adhesives comprise a polymerized product of a syrup polymer mixture comprising:
(a) from 2 to 20 parts by weight of a first component comprising a solute polymer having pendant reactive electrophilic or nucleophilic functional groups:
(b) from 0.01 to 10.00 parts by weight of a second component having a plurality of co-reactive electrophilic or nucleophilic functional groups;
(c) from 70.00-97.99 parts by weight of a third component comprising polymerizable, ethylenically-unsaturated monomers selected from acrylic acid esters of non-tertiary alkyl alcohols containing 1-14 carbon atoms (i.e. xe2x80x9cacrylate monomersxe2x80x9d).
This invention also provides a polymerizable syrup polymer comprising the syrup polymer and photoinitiator.
The first solute polymer, as well as the second solute polymer (if used in lieu of the polyfunctional compound) comprises
(1) from 75.00 to 99.99 parts by weight of polymerized monomer units derived from acrylic acid esters of non-tertiary alkyl alcohols containing 1-14 carbon atoms;
(2) from 0.01 to 5.00 parts by weight of a polymerized monomer units derived from an ethylenically-unsaturated monomer possessing co-reactive functional groups; (i.e. xe2x80x9cfunctional monomersxe2x80x9d);
(3) from 0 to 10 parts by weight of at least one polar monomer; (i.e. xe2x80x9cpolar monomersxe2x80x9d) and
(4) from 0 to 10 parts by weight of other monomers (described below).
The second component of the syrup polymer composition (b) may be a second solute polymer having co-reactive functional groups, or may be a polyfunctional compound having a plurality of co-reactive functional groups. Where a second solute polymer is used, the polymer may be prepared in situ in the syrup polymer mixture or may be separately prepared and added to the syrup polymer mixture. The pressure sensitive adhesive of the invention results from polymerization of the syrup polymer composition and crosslinking formed by reaction of the reactive and co-reactive functional groups.
Monomers that are useful and that comprise the major portion of the first and second solute polymers, and the third component solvent monomers are predominantly alkyl acrylate esters. Alkyl acrylate ester monomers useful in the invention include straight-chain, cyclic, and branched-chain isomers of alkyl esters containing C1-C14 alkyl groups. Due to Tg and sidechain crystallinity considerations, preferred alkyl acrylate esters are those having from C5-C12 alkyl groups, although use of C1-C4 and C13-C14 alkyl groups are also useful if the combinations provide a molecule averaged number of carbon atoms between C5 and C12. Useful specific examples of alkyl acrylate esters include: methyl acrylate, ethyl acrylate, n-propyl acrylate, 2-butyl acrylate, iso-amyl acrylate, n-hexyl acrylate, n-heptyl acrylate, isobornyl acrylate, n-octyl acrylate, iso-octyl acrylate, 2-ethylhexyl acrylate, iso-nonyl acrylate, decyl acrylate, undecyl acrylate, dodecyl acrylate, tridecyl acrylate, and tetradecyl acrylate. Most preferred acrylate esters include iso-octyl acrylate, 2-ethylhexyl acrylate, and isobomyl acrylate.
Useful functional monomers include those unsaturated aliphatic, cycloaliphatic, and aromatic compounds having up to about 36 carbon atoms that include a functional group capable of further reaction, such as a hydroxyl, amino, azlactone, oxazolinyl, 3-oxobutanoyl (i.e., acetoacetyl), carboxyl, isocyanato, epoxy, aziridinyl, acyl halide, vinyloxy, or cyclic anhydride group.
Preferred functional monomers have the general formula 
wherein R1 is hydrogen, a C1 to C4 alkyl group, or a phenyl group, preferably hydrogen or a methyl group; R2 is a single bond or a divalent linking group that joins an ethylenically unsaturated group to functional group A and preferably contains up to 34, preferably up to 18, more preferably up to 10, carbon and, optionally, oxygen and nitrogen atoms and, when R2 is not a single bond, is preferably selected from 
in which R3 is an alkylene group having 1 to 6 carbon atoms, a 5- or 6-membered cycloalkylene group having 5 to 10 carbon atoms, or an alkylene-oxyalkylene in which each alkylene includes 1 to 6 carbon atoms or is a divalent aromatic group having 6 to 16 carbon atoms; and A is a functional group, capable of reaction with a co-reactive functional group (which is part of an unsaturated monomer) to form a covalent bond, preferably selected from the class consisting of hydroxyl, amino (especially secondary amino), carboxyl, isocyanato, aziridinyl, epoxy, acyl halide, vinyloxy, azlactone, oxazolinyl, acetoacetyl, and cyclic anhydride groups.
Representative hydroxyl group-substituted functional monomers include the hydroxyalkyl (meth)acrylates and hydroxyalkyl (meth)acrylamides such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-chloro-2-hydroxypropylmethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylamide, 4-hydroxycyclohexyl (meth)acrylate, 3-acryloyloxyphenol, 2-(4-acryloyloxyphenyl)-2-(4-hydroxyphenyl)propane (also called bisphenol A monoacrylate), 2-propyn-1-ol, and 3-butyn-1-ol.
Representative amino group-substituted functional monomers include 2-methyl aminoethyl methacrylate, 3-aminopropyl methacrylate, 4-aminocyclohexyl methacrylate, N-(3-aminophenyl)acrylamide, 4-aminostyrene, N-acryloylethylenediamine, and 4-aminophenyl-4-acrylamidophenylsulfone.
Representative azlactone group-substituted functional monomers include 2-ethenyl-1,3-oxazolin-5-one; 2-ethenyl-4-methyl-1,3-oxazolin-5-one; 2-isopropenyl-1,3-oxazolin-5-one; 2-isopropenyl-4-methyl-1,3-oxazolin-5-one; 2-ethenyl-4,4-dimethyl-1,3-oxazolin-5-one; 2-isopropenyl-4,4-dimethyl-1,3-oxazolin-5-one; 2-ethenyl-4-methyl-4-ethyl-1,3-oxazolin-5-one; 2-isopropenyl-3-oxa-1-aza[4.5]spirodec-1-ene-4-one; 2-ethenyl-5,6-dihydro-4H-1,3-oxazin-6-one; 2-ethenyl-4,5,6,7-tetrahydro-1,3-oxazepin-7-one; 2-isopropenyl-5,6-dihydro-5,5-di(2-methylphenyl)-4H-1,3-oxazin-6-one; 2-acryloyloxy-1,3-oxazolin-5-one; 2-(2-acryloyloxy)ethyl-4,4-dimethyl-1,3-oxazolin-5-one; 2-ethenyl-4,5-dihydro-6H-1,3-oxazin-6-one, and 2-ethenyl-4,5-dihydro-4,4-dimethyl-6H-1,3-oxazin-6-one.
Representative oxazolinyl group-substituted functional monomers include 2-vinyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-(5-hexenyl)-2-oxazoline, 2-acryloxy-2-oxazoline, 2-(4-acryloxyphenyl)-2-oxazoline, and 2-methacryloxy-2-oxazoline.
Representative acetoacetyl group-substituted functional monomers include 2-(acetoacetoxy)ethyl (meth)acrylate, styryl acetoacetate, isopropenyl acetoacetate, and hex-5-enyl acetoacetate.
Representative carboxyl group-substituted functional monomers include (meth)acrylic acid, 3-(meth)acryloyloxy-propionic acid, 4-(meth)acryloyloxy-butyric acid, 2-(meth)acryloyloxy-benzoic acid, 3-(meth)acryloyloxy-5-methyl benzoic acid, 4-(meth)acryloyloxymethyl-benzoic acid, phthalic acid mono-[2-(meth)acryloyloxy-ethyl]ester, 2-butynoic acid, and 4-pentynoic acid.
Representative isocyanate group-substituted functional monomers include 2-isocyanatoethyl (meth)acrylate, 3-isocyanatopropyl (meth)acrylate, 4-isocyanatocyclohexyl (meth)acrylate, 4-isocyanatostyrene, 2-methyl-2-propenoyl isocyanate, 4-(2-acryloyloxyethoxycarbonylamino)phenylisocyanate, allyl 2-isocyanatoethylether, and 3-isocyanato-1-propene.
Representative epoxy group-substituted functional monomers include glycidyl (meth)acrylate, thioglycidyl (meth)acrylate, 3-(2,3-epoxypropxy)phenyl (meth)acrylate, 2-[4-(2,3-epoxypropoxy)phenyl]-2-(4-acryloyloxy-phenyl)propane, 4-(2,3-epoxypropoxy)cyclohexyl (meth)acrylate, 2,3-epoxycyclohexyl (meth)acrylate, and 3,4-epoxycyclohexyl (meth)acrylate.
Representative aziridinyl group-substituted functional monomers include N-(meth)acryloylaziridine, 2-(1-aziridinyl)ethyl (meth)acrylate, 4-(1-aziridinyl)butyl (meth)acrylate, 2-[2-(1-aziridinyl)ethoxy]ethyl (meth)acrylate, 2-[2-(1-aziridinyl)ethoxycarbonylamino]ethyl (meth)acrylate, 12-[2-(2,2,3,3-tetramethyl-1-aziridinyl)ethoxycarbonylamino]dodecyl (meth)acrylate, and 1-(2-propenyl)aziridine.
Representative acyl halide group-substituted functional monomers include (meth)acryloyl chloride, xcex1-chloroacryloyl chloride, acryloyloxyacetyl chloride, 5-hexenoyl chloride, 2-(acryloyloxy) propionyl chloride, 3-(acryloylthioxy) propionoyl chloride, and 3-(N-acryloyl-N-methylamino) propionoyl chloride.
Representative vinyloxy group-substituted functional monomers include 2-(ethenyloxy)ethyl (meth)acrylate, 3-(ethynyloxy)-1-propene, 4-(ethynyloxy)-1-butene, and 4-(ethenyloxy)butyl-2-acrylamido-2,2-dimethylacetate.
Representative anhydride group-substituted functional monomers include maleic anhydride, acrylic anhydride, itaconic anhydride, 3-acryloyloxyphthalic anhydride, and 2-methacryloxycyclohexanedicarboxylic acid anhydride.
It will be understood in the context of the above description of the first and second solute polymers, that the ethylenically-unsaturated monomer possessing a reactive functional group (xe2x80x9creactive monomerxe2x80x9d) is chosen such that the first and second components are mutually co-reactive so that the first solute polymer has a pendant functional group that is co-reactive with the pendant functional group of the second component. The reactive and co-reactive functional groups form a crosslink between the first and second components by forming a linking group between the electrophilic and nucleophilic functional group pairs, and may include reactions commonly referred to as displacement, condensation and addition reactions, rather than polymerization of ethylenically-unsaturated groups.
While it is within the scope of the invention to employ nucleophile-electrophile combinations that react by displacement of some leaving group and creation of a by-product molecule, the removal of by-products requires an additional processing step. It is preferred that the nucleophile-electrophile combinations react by an addition reaction in which no by-product molecules are created, and the exemplified reaction partners react by this preferred mode. Exemplary combinations include hydroxyl or amino functional groups reacting with azlactone-, isocyanate-, and anhydride-functional groups and carboxyl groups reacting with isocyanate- and oxazoline-functional groups.
To aid in the understanding of this interaction between reactive first and co-reactive second functional groups, Table 1 summarizes some possible combinations of functional groups, using carboxyl and hydroxyl groups as representative examples. Those skilled in the art will readily recognize how other previously described functional groups also can be used to form covalent linking groups.
In Table I, each R12 is independently hydrogen, an alkyl group having 1 to 4 carbon atoms, or a phenyl group. R13 and R14 are independently hydrogen or an alkyl group containing from 1 to about 4 carbon atoms, although R13 and R14 preferably are not both alkyl groups.
Representative examples of free-radically polymerizable polar monomers having at least one ethylenically unsaturated polymerizable group which are copolymerizable with acrylate and functional monomers include strongly polar copolymerizable monomers including but not limited to those selected from the group consisting of substituted (meth)acrylamides, N-vinyl pyrrolidone, N-vinyl caprolactam, acrylonitrile, tetrahydrofurfuryl acrylate, acrylamides, and mixtures thereof, and the like.
The selection of the other monomers useful in preparing the functional syrup polymer(s) (of the first and second components) and to provide additional solvent monomers (third component) for the syrup polymer(s) is done in such a manner that the ultimate crosslinked pressure sensitive adhesive has sufficient conformability, tack, and adhesion to form a bond to a substrate at room temperature. One measure of a psa""s ability to conform to an substrate sufficiently at room temperature and to form an adhesive bond is the material""s glass transition temperature (Tg). A useful, guiding principal is that a psa interpolymer should have a Tg of xe2x88x9215xc2x0 C. (258xc2x0 K) or lower in order for effective adhesive application at room temperature. A useful predictor of interpolymer Tg for specific combinations of various monomers can be computed by application of Equation (1) (obtained from W. R. Sorenson and T. W. Campbell""s text entitled xe2x80x9cPreparative Methods of Polymer Chemistryxe2x80x9d, Interscience: New York (1968), p. 209).                               1                      T            g                          =                              ∑                          n              =              1                        1                    ⁢                                    W              1                                                      T                g                            ⁢              i                                                          (                  Equation          ⁢                      xe2x80x83                    ⁢          1                )            
wherein
Tg=Glass transition temperature in degrees Kelvin for the interpolymer
Tgi=Glass transition temperature in degrees Kelvin for the homopolymer of the ith monomer
Wi=Weight fraction of the ith monomer
Specific values for Tg""s of appropriate homopolymers can be obtained from P. Peyser""s chapter in xe2x80x9cPolymer Handbookxe2x80x9d, 3rd edition, edited by J. Brandrup and E. H. Immergut, Wiley: New York (1989), pp. VI-209 through VI-277.
Useful xe2x80x9cother monomersxe2x80x9d include vinyl monomers such as vinyl acetate, styrenes, and alkyl vinyl ethers; and alkyl methacrylates. Useful xe2x80x9cother monomersxe2x80x9d may also include various polyunsaturated monomers, including addition products or copolymers or oligomers comprising two different functional monomers (as defined previously) such that the product/copolymer/oligomer exhibits the functionality of both of the constituent starting materials/monomers. Examples of useful polyfunctional compounds include allyl, propargyl and crotyl (meth)acrylates; ethylene di(meth)acylate; 1,6-hexanediol diacrylate (HDDA), trimethylol propane triacrylate; pentaerythritol triacrylate; allyl-2-acrylamido-2,2-dimethyl acetate and the like.
Useful polyfunctional compounds (as the second component) have an average functionality (average number of functional groups per molecule) of greater than one, preferably greater than two and most preferably greater than 3. The functional groups are chosen to be co-reactive with the pendant functional groups on the first solute polymer, and may be nucleophilic or electrophilic. Useful functional groups include those described for the first solute polymer and include, but are not limited to hydroxyl, amino (especially secondary amino), carboxyl, isocyanato, aziridinyl, epoxy, acyl halide, vinyloxy, azlactone, oxazolinyl, acetoacetone, and cyclic anhydride groups. Useful polyfunctional compounds have the general formula R-(Z)n where Z is a functional group, n is greater than 1 and R is an organic radical having a valency of n. Preferably R is an alkyl radical of valency n which may be linear or branched. Most preferred functional groups for polyfunctional compounds are those having hydroxyl, isocyanato, aziridinyl and azlactone functional groups.
Dendritic polymers are preferred polyfunctional compounds and include any of the known dendritic architectures including dendrimers, regular dendrons, dendrigrafts, and hyperbranched polymers. Dendritic polymers are polymers with densely branched structures having a large number of end reactive groups. A dendritic polymer includes several layers or generations of repeating units which all contain one or more branch points. Dendritic polymers, including dendrimers and hyperbranched polymers, can be prepared by condensation, addition, or ionic reactions of monomeric units having at least two different types of reactive groups.
Dendritic polymers are comprised of a plurality of dendrons that emanate from a common core, which core usually comprises a group of atoms. Dendritic polymers generally consist of peripheral surface groups, interior branch junctures having branching functionalities greater than or equal to two, and divalent connectors that covalently connect neighboring branching junctures.
Dendrimers can be prepared by convergent or divergent synthesis. Divergent synthesis of dendrimers involves a molecular growth process which occurs through a consecutive series of geometrically progressive step-wise additions of branches upon branches in a radially outward molecular direction to produce an ordered arrangement of layered branch generations, in which each macromolecule includes a core generation, one or more layers of internal generations, and an outer layer of surface generations, wherein each of the generations includes a single branch juncture. The generations can be the same or different in chemical structure and branching functionality. The surface branch generations may contain either chemically reactive or passive functional groups. Chemically reactive surface groups can be used for further extension of dendritic growth or for modification of dendritic molecular surfaces. The chemically passive groups may be used to physically modify dendritic surfaces, such as to adjust the ratio of hydrophobic to hydrophilic terminals. Convergent synthesis of dendrimers involves a growth process which begins from what will become the surface of the dendrimers and progresses radially in a molecular direction toward a focal point or core.
Dendrons and dendrimers may be ideal or non-ideal, i.e., imperfect or defective. Imperfections are normally a consequence of either incomplete chemical reactions or unavoidable competing side reactions.
Hyperbranched polymers can be prepared by one-pot polymerization reaction of a single type of monomer having a single reactive group of a first type (B) and a plurality (y) of reactive groups of a second type (A), i.e., a Bxe2x80x94Ay type monomer, which is initiated by a core having a plurality (x) of the A type reactive groups, wherein A groups can react with B groups but not other A groups, and B groups cannot react with other B groups. The one-pot synthesis method for hyperbranched polymers is simpler and less expensive than the divergent and convergent synthesis methods for dendrimers. However, the one-pot synthesis method lacks reaction control, which leads to more polydisperse products with larger deviations from ideal dendron structure.
Hyperbranched polymers are dendritic polymers that contain high levels of non-ideal irregular branching arrays as compared with the more nearly perfect regular structure dendrimers. Specifically, hyperbranched polymers contain a relatively high number of irregular branching arrays in which not every repeat unit contains a branch juncture. Consequently, hyperbranched polymers may be viewed as intermediate between linear polymers and dendrimers. Yet they are dendritic because of their relatively high branch-juncture content per individual macromolecule.
The preparation and characterization of dendrimers, dendrons, dendrigrafts, and hyperbranched polymers, is well known. Examples of dendrimers and dendrons, and methods of synthesizing the same are set forth in U.S. Pat. Nos. 4,507,466; 4,558,120; 4,568,737; 4,587,329; 4,631,337; 4,694,064; 4,713,975; 4,737,550; 4,871,779 and 4,857,599. Examples of hyperbranched polymers and methods of preparing the same are set forth, for example, in U.S. Pat. No. 5,418,301. Some dendritic polymers are also commercially available. For example, 3- and 5-generation hyperbranched polyester polyols may be obtained from Perstorp Polyols, Inc., Toledo, Ohio.
More generally, dendritic polymers or macromolecules are characterized by a relatively high degree of branching (DB), which is defined as the number average fraction of branching groups per molecule, i.e., the ratio of terminal groups plus branch groups to the total number of terminal groups, branch groups and linear groups. For dendrimers, the degree of branching is one. For linear polymers the degree of branching approaches zero. Hyperbranched polymers have a degree of branching that is between that of linear polymers and ideal dendrimers. The dendritic polymers used in this invention preferably have a degree of branching which is at least equal to 0.1, more preferably greater than 0.4, and most preferably greater than 0.5.
As previously described, the composition of the present invention comprises a first solute polymer with a plurality of pendant reactive functional groups, a second component comprising co-reactive functional groups, a monomer mixture and optionally an initiator. Formation of the composition (i.e., the bringing together of the three components, and the photoinitiator) can be accomplished in several ways. Preferably, they are brought together after the first solute polymer has been separately prepared.
The first solute polymer can be prepared (e.g., by solution polymerization followed by isolation) and then added to a separately prepared second and third component mixture. Depending on the type of coating process to be used, the relative amounts of the solute polymer(s) and third monomer component can vary greatly. For example, where the coating is to be done by a solvent or hot-melt process, the relative amount of the first and second components preferably is relatively high. However, where coating is to be done by a syrup application process, the relative amount of polymer preferably is low.
The coatable syrup polymer is prepared by combining the three component composition containing the first solute polymer, the second component and the third component monomer. Polymerization may be necessary to achieve a thickened solution exhibiting a coatable viscosity of from about 500-10,000 cPs at 22xc2x0 C., more preferably from about 750 to 7500 cPs.
In general, the order of addition is conducted so as to minimize the reaction between the reactive and co-reactive functional groups prior to coating and thus maximize the useful shelf life or xe2x80x9copen timexe2x80x9d, i.e. the time during which the adhesive is applied to a first substrate (such as a tape backing) and remains sufficiently tacky to effect a bond between the first substrate and a second substrate. Once the open time has been exceeded, the second substrate cannot be readily bonded to the first substrate. Long open times are generally preferred. Shelf life refers to the amount of time the syrup polymer may be stored without premature gelation.
To avoid premature gelation it is generally advantageous to avoid having both reactive and co-reactive groups on the first polymer component, or having both reactive and co-reactive groups on the second component. If the reactive and co-reactive groups are not highly reactive, i.e. do not react at appreciable rates at either ambient temperature or in the absence of a catalyst, then one may accommodate the reactive and co-reactive groups on the first polymer component. Similarly, if the relative concentrations of either the reactive or co-reactive function groups are low, then the two will not react at appreciable rates and gelation may be avoided. However, where the two do react at significant rates, gelation may be avoided by adding one of the components just prior to coating.
The syrup polymer solution may be coated onto backings at useful and relatively time-stable thicknesses ranging from 25-500 micrometers or more. Stable thicknesses are necessary to maintain the desired coating thickness prior to further polymerization and crosslinking of the syrup polymer to form the crosslinked pressure sensitive adhesives. Coating can be accomplished by any conventional means such as roller, dip, knife, or extrusion coating. The use of a composition of a coatable viscosity provides the advantage of allowing the remaining monomer(s) to be polymerized after they have been applied to a substrate.
A preferred method of preparing a pressure sensitive adhesive article comprises partially polymerizing the novel syrup polymer composition to a useful coating viscosity, coating the partially polymerized syrup polymer composition onto a substrate (such as a tape backing) and further polymerizing the syrup polymer. Partial polymerization provides a coatable solution of the first and second solute polymers in one or more third component solvent monomers.
For syrup application processing, a preferred monomer mixture (third component) comprises 50 to 100 pbw of one or more acrylate ester monomers, 0 to 50 pbw of one or more polar monomers, and, per 100 pbw of the acrylate ester and polar monomers, 0 to about 20 pbw of a functional monomer, and 0 to about 20 pbw of xe2x80x9cother monomersxe2x80x9d, including 0 to 0.5 pbw of polyunsaturated monomers.
The polymerizations may be conducted in the presence or preferably in the absence of suitable solvents such as ethyl acetate, toluene and tetrahydrofuran which are unreactive with the functional groups of the components of the syrup polymer.
Polymerization of the monomer components may be used to form the second solute polymer in situ. This method of forming the composition of the present invention has the advantage of allowing for compositions in which very high molecular weight polymers are dissolved in a monomer mixture.
Polymerization can be accomplished by exposing the syrup polymer composition to energy in the presence of a photoinitiator. Energy activated initiators may be unnecessary where, for example, ionizing radiation is used to initiate polymerization. These photoinitiators can be employed in concentrations ranging from about 0.0001 to about 3.0 pbw, preferably from about 0.001 to about 1.0 pbw, and more preferably from about 0.005 to about 0.5 pbw, per 100 pbw of the third component solvent monomer.
A preferred method of preparation of the coatable syrup polymer is photoinitiated free radical polymerization. Advantageously, a photoinitiated process generally generates enough heat to effect the reaction between the reactive and co-reactive functional groups to crosslink the polymers and produce a pressure sensitive adhesive. Additional advantages of the photopolymerization method are that 1) heating the monomer solution is unnecessary and 2) photoinitiation is stopped completely when the activating light source is turned off.
Polymerization to achieve a coatable viscosity may be conducted such that the conversion of monomers to polymer is up to about 30%. Polymerization can be terminated when the desired conversion and viscosity have been achieved by removing the light source and by bubbling air (oxygen) into the solution to quench propagating free radicals. The solute polymer(s) may be prepared conventionally in a non-monomeric solvent and advanced to high conversion. When solvent is used, the solvent may be removed (for example by vacuum distillation) either before or after formation of the syrup polymer. While an acceptable method, this procedure involving a highly converted functional polymer is not preferred because an additional solvent removal step is required, another material may be required (the non-monomeric solvent), and dissolution of the high molecular weight, highly converted solute polymer in the monomer mixture may require a significant period of time.
If so desired, the extent of polymerization can be monitored by measuring the refractive index of the composition/viscoelastomeric material especially in bulk. Refractive index changes linearly with respect to conversion. This monitoring method is commonly applied in polymerization kinetics work. See discussions about the method in, for example, G. P. Gladyshev and K. M. Gibov, Polymerization at Advanced Degrees of Conversion, Keter Press, Jerusalem (1970).
Useful photoinitiators include benzoin ethers such as benzoin methyl ether and benzoin isopropyl ether; substituted acetophenones such as 2,2-diethoxyacetophenone, available as Irgacure(trademark) 651 photoinitiator (Ciba-Geigy Corp.; Ardsley, N.Y.), 2,2-dimethoxy-2-phenyl-1-phenylethanone, available as Esacure(trademark) KB-1 photoinitiator (Sartomer Co.; West Chester, Pa.), and dimethoxyhydroxyacetophenone; substituted (xcex1-ketols such as 2-methyl-2-hydroxy propiophenone; such as 2-naphthalene-sulfonyl chloride; such as 1-phenyl-1,2-propanedione-2-(O-ethoxy-carbonyl)oxime. Particularly preferred among these are the substituted acetophenones.
Preferred photoinitiators are photoactive compounds that undergo a Norrish I cleavage to generate free radicals that can initiate by addition to the acrylic double bonds. Norrish type 1 photocrosslinkers, especially xcex1-cleaving type photoinitiators, are preferred. The photoinitiator can be added to the mixture to be coated after the first solute polymer has been formed (i.e., photoinitiator can be added to the syrup polymer mixture. Such polymerizable photoinitiators are described, for example, in U.S. Pat. Nos. 5,902,836 (Babu et al.) and 5,506,279 (Babu et al.), the disclosures of which are herein incorporated by reference.
The syrup polymer composition and the photoinitiator may be irradiated with activating UV radiation to polymerize the monomer component(s). UV light sources can be of two types: 1) relatively low light intensity sources such as Blacklights which provide generally 10 mW/cm2 or less (as measured in accordance with procedures approved by the United States National Institute of Standards and Technology as, for example, with a UVIMAP(trademark) UM 365 L-S radiometer manufactured by Electronic Instrumentation and Technology, Inc., in Sterling, Va.) over a wavelength range of 280 to 400 nanometers and 2) relatively high light intensity sources such as medium pressure mercury lamps which provide intensities generally greater than 10 mW/cm2, preferably between 15 and 450 mW/cm2. Where actinic radiation is used to fully or partially polymerize the syrup polymer composition, high intensities and short exposure times are preferred. For example, an intensity of 600 mW/cm2 and an exposure time of about 1 second may be used successfully. Intensities can range from about 0.1 to about 150 mW/cm2, preferably from about 0.5 to about 100 mW/cm2, and more preferably from about 0.5 to about 50 mW/cm Accordingly, relatively thick coatings (e.g., at least about 0.05 mm, preferably at least about 0.10 mm, more preferably at least about 0.15 mm thick) can be achieved when the extinction coefficient of the photoinitiator is low. Coatings from of 0.5 up to 2 mm thick are possible and are within the scope of the present invention. Such photoinitiators preferably are present in an amount of from 0.1 to 1.0 pbw per 100 pbw of the syrup polymer composition.
The degree of conversion can be monitored during the irradiation by measuring the index of refraction of the polymerizing medium as previously described. Useful coating viscosities are achieved with conversions (i.e. the percentage of available monomer polymerized) in the range of up to 30%, preferably 2-20%, more preferably from 5-15%, and most preferably from 7-12%. The molecular weight (weight average) of the solute polymer(s) is at least 100,000, preferably at least 500,000, and more preferably at least 1,000,000.
When preparing a psa of the invention, it is expedient for the photoinitiated polymerization reactions to proceed to virtual completion, i.e., depletion of the monomeric components, at temperatures less than about 70xc2x0 C. (preferably at 50xc2x0 C. or less) with reaction times less than 24 hours, preferably less than 12 hours, and more preferably less than 6 hours. These temperature ranges and reaction rates obviate the need for free radical polymerization inhibitors, which are often added to acrylic systems to stabilize against undesired, premature polymerization and gelation. Furthermore, the addition of inhibitors adds extraneous material that will remain with the system and inhibit the desired polymerization of the syrup polymer and formation of the crosslinked pressure sensitive adhesives of the invention. Free radical polymerization inhibitors are often required at processing temperatures of 70xc2x0 C. and higher for reaction periods of more than about 6 hours.
Use of a stoichiometric excess of a component containing a functional group or a co-reactive functional group may be useful to achieve sufficient reaction between functional groups under the above specified conditions. Stoichiometric excesses of even 10-fold represent minor amounts on a comparative weight basis to whole the pressure sensitive adhesive. Catalysts may be used to enhance rates of addition reaction between reactive and co-reactive functional groups and to effect the crosslinking of the syrup polymer components. Metal catalysts such as dibutyltin dilaurate and dibutyltin diacetate are effective with alcohol-isocyanate combinations. Strong acids such as ethanesulfonic acid and methanesulfonic acid are useful with azlactone-alcohols and with the anhydride-alcohols. Effective concentrations of the catalytic agents are from 0.01 to 5.00 weight percent based on the concentration of the stoichiometrically limiting reactant.
In addition to the ingredients mentioned above, the syrup polymer composition may include certain other materials such as pigments, tackifiers and reinforcing agents. However, the addition of any such material adds complexity and hence expense to an otherwise simple, straightforward, economical composition and process and is not preferred except to achieve specific results.